feat(setup): `upstream` remote when running script

This is so that users can keep track of changes to the plugin while
maintaining their own changes.

CLOSES #14

setup.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/sh
+
 sed "s/$1/$2/g" "colors/$1.lua" > "colors/$2.lua"
 rm "colors/$1.lua"
 
@@ -10,6 +11,9 @@ mv "lua/$1.lua" "lua/$2.lua"
 sed "s/$1/$2/g" "README.md" > "README_2.md"
 mv "README_2.md" "README.md"
 
+git remote add upstream https://github.com/Iron-E/nvim-highlite.git
+git fetch --all
+
 echo "Run 'set rtp+=$(pwd)' from within Vim to include this folder in your runtime path."
 echo ""
 echo "You must do this before sourcing your colorscheme, unless you are using a plugin manager."